Conventional ion-mobility spectrometry (IMS) instruments and IMS-mass spectrometry (IMS-MS) instruments typically include an ion trap and an ion mobility separation device. An IMS-MS instrument further includes a mass analyzer, and is capable of simultaneously producing mass spectral data and ion mobility data. For example, IMS-MS instruments combined with liquid chromatography (LC) techniques provide another dimension in separation, and may simplify analysis for very complex analytical and biological samples. However, a conventional IMS or IMS-MS instrument has a useful dynamic range which is typically limited on both the low and high ends. For example, the low end may be limited due to electronic, chemical or digitization noise, and the high end may be limited by saturation of ion conversion or electronics. So, the ion detection system will have a useful “middle portion” of its range (useful dynamic range) in which the response to increasing ion intensity (abundance) is sufficiently linear for the purposes of the measurement, or stated differently, is sufficiently proportional to the ion abundance for the purposes of the measurement. The limited useful dynamic range limits the ability to analyze different ion concentrations at the same time. It is desirable to improve the effective dynamic range of IMS or IMS-MS instrument.
Attempts have been made to improve dynamic range. For example, U.S. Pat. No. 6,900,431 to Belov et al., issued May 31, 2005, discloses a system having an encoded pulsing scheme with post process decoding of the obtained data. However, the typical dynamic range extension is still limited, increasing only by a factor of about 10 or less, in practice. In another technique, disclosed by Ibrahim et al., Automated Gain Control for Orthogonal Time-of-Flight Mass Spectrometry, ANAL. CHEM. 2008, 80, pp. 5367-5376, automated gain control is used, such that trapping time in the ion trap is adjusted based on the total ion current based on the previous measurement. However, when a small ion population needs to be detected in the presence of a much larger population, this technique is inadequate.